Experimental measurements, interpreted with mean field theories, provide quantitative understanding of the stability of colloidal particles coated with a highly asymmetric diblock copolymer. Adsorption of poly((dimethylamino)ethyl methacrylate)/poly(n-butyl methacrylate) onto colloidal silica particles from 2-propanol, which is nonselective and a Theta solvent for poly(n-butyl methacrylate), yields a very high surface coverage and a highly stretched brush. With increasing temperature, the adsorbed chains swell more than those in the bulk. Upon cooling, the dispersions exhibit thermodynamic stability well below the Theta temperature of the nonadsorbed block. A simple mean-field theory rationalizes the stability as a consequence of the high surface coverage, which is attributed in turn to strong segmental adsorption of the short anchor block and negligible excluded volume interactions among the stabilizing chains.